Think of a world that changed fundamentally after Marx died. His books became forgotten, and never received circulation beyond the German language. In that world, there has been no Soviet Union. There has not even been an attempt at socialism anywhere, because there was nothing to drive it.
We don’t live in that world. We live in one where all those things existed, and continue to exist, and continue to effect influence on us, traveling through time with ourselves as the catalyst.
If Marx wouldnt have been, another person would have come up with the same or an equivalent of his works. What we know as Marxism would never have not existed the way history was progressing.